The radius and ulna together form the bony structure of the forearm. These bones are linked and articulate with one another at both their proximal ends (at the elbow) and distal ends (at the wrist). The distal radioulnar joint (DRUJ) is a pivot joint adjacent the wrist, with the head of the ulna and sigmoid notch of the radius bone interacting so that the distal end of the radius articulates in pronation and supination on the distal head of the ulna. As seen in FIG. 1, the distal portion of the ulna U (i.e. that portion closer to the wrist and hand) has a head H with an articular surface AS and a styloid process SP, and the distal portion of the radius bone R includes a sigmoid notch N on its medial side. In a normal DRUJ, articular surface AS engages sigmoid notch N, and the DRUJ permits the familiar range of motion in pronation, supination and pivoting about axes substantially perpendicular to the radius and ulna bones. Fractures of the distal radius and/or ulna, other injuries of the forearm, diseases affecting the tissues or other maladies may affect the distal ends of the radius and/or ulna and cause rotational instability of the DRUJ. Ulnar styloid process fractures, and fractures into the distal radioulnar joint can occur with such injuries or other trauma. Fracture or dislocation involving the DRUJ can result in a loss of forearm rotation related to either instability or incongruity between the mating parts of the distal radius and ulna.
With severe injuries to the DRUJ or other loss of its stability, loss of the ability to rotate the forearm (pronation and supination) as well as weakness in the individual's grip and/or pinch can occur. Treatments for damage to the distal radius or ulna and/or correction of the DRUJ have been proposed using hemiarthroplasty, which features resection of the head of the ulna and replacement with a prosthesis. Such prostheses contact the sigmoid notch of the radius bone directly to allow some pivoting, in a way approximating the natural configuration of the joint. However, contact between a prosthesis and natural bone can unnaturally wear the bone tissue, potentially leading to damage to the sigmoid notch or other part of the radius bone, an arthritis condition, or other degenerative change. Accordingly, reinforcements for the sigmoid notch in the form of a plate or similar implant covering the notch have been used. These items are designed to keep the ulnar prosthesis applied in hemiarthroplasty from contacting bone. That solution derives from a desire to maintain the notch or similar structure in the radius bone. It also derives from an interest in maintaining most or all of the head of the radius bone intact.
Other types implants have been proposed to replace surfaces of both the ulna and the radius bones. However, existing implants still suffer significant lack of stability and/or mobility in the joint. Accordingly, there is a need for a prosthetic device that provides greater stability and mobility in repairing a damaged DRUJ.